Noble metals and their alloys have important applications as catalysts for many chemical and electrochemical reactions in both industry and research. Metal and alloy nanoparticles on a support material are one common practice for their use as a catalyst. The use of a support material is for separation and stabilization of the nanoparticles from agglomeration and sintering. The main driving force to make the noble metals and their alloys into nanoparticles is the high cost of the precious metals. Primarily, only surface atoms are involved in catalysis, therefore, the overall cost of catalysts can be largely reduced by making the particles smaller.
Noble metal and alloy nanoparticles with specific morphologies have been found to exhibit improved catalytic properties, such as activity, selectivity, and durability, in many reactions. Therefore, by manipulating the morphology of noble metal alloy nanoparticles, the catalytic properties of the catalyst can be improved, and their usage can be decreased. Many different methods have been explored for preparing noble metal alloy catalysts with specific morphologies. In the conventional impregnation method an active metal precursor is dissolved in an aqueous or organic solution and then the metal-containing solution is added to a catalyst support containing a pore volume equal to the volume of the metal-containing solution that is added. Capillary action draws the solution into the pores. Solution added in excess of the support pore volume causes the solution transport to change from a capillary action process to a diffusion process, which is much slower. The catalyst can then be dried and calcined to drive off the volatile components within the solution, depositing the metal on the catalyst surface.
The impregnation method is one of the most robust techniques in making a supported noble metal nanoparticle and has been broadly adopted for both industrial production and fundamental research in laboratories. However, there is a lack of control over the size of the metal particles, the morphology of the metal particles, and the uniformity of the metal particles when the impregnation method is used. The final product often contains a mixture of pseudo-spherical particles of different sizes, which is not ideal.
In other methods, noble metal particles are synthesized in a solution and are then placed onto a support material. The solution typically contains capping agents, which are chemical molecules which can strongly adsorb to the surface of materials, such as nanoparticles. The adsorption of capping agents to growing nanoparticles can alter their growth behavior and thus influence the morphology of the growing particles. However, these synthetic techniques have obvious limitations and can hardly be transformed into real application, especially for applications as important as making catalysts. The main limitation of all current approaches for the synthesis of shaped metal particles is that they are based on wet chemistry techniques that have complex procedures. The complexity and rigid requirements on the reaction conditions largely limit the capability for large-scale production of the shaped metal nanoparticles. In addition, the heavy usage of different organic species adds to the production costs and contaminates the surface of the nanoparticles. In order for the shaped metal nanoparticle to be used as a catalyst, further steps must be taken to clean the surface of the synthesized noble metal nanoparticle.
Thus, there is a need in the art for noble metal and alloy catalysts with controlled particle morphology and methods for making them that eliminate the contamination problem caused by the use of organic capping agents and which can simplify the overall synthetic procedure.